1. Field of the Invention
The present invention relates to a speaker system using a plurality of speaker units.
2. Description of the Prior Art
Recently, as digital signal processing technology has been dramatically advanced, the electrical signals supplied to speaker systems have notably improved in quality, and accordingly there is an increased demand for a speaker system capable of reproducing sound at a high sound quality. For high quality sound reproduction, the speaker system must be free from large fluctuations in the sound pressure frequency characteristics depending on the sound listening position. For this purpose, it is necessary to minimize the effects of reflected sound from the walls and floor of the listening room, and it is preferable to develop a speaker system in which the directivity is controlled in a wide frequency band so that sound is not radiated to areas other than the listening position. In the low frequency range, however, the wavelength of the sound wave is long and the directivity is nondirectional, so that it is difficult to control a directivity. Especially in the speaker system using one speaker unit for bass reproduction, since the directivity is determined by the size of the diaphragm of the speaker unit, it is difficult to control the directivity in the low frequency range.
As the means for solving this problem, there is a commercial speaker system which has four speaker units for bass reproduction disposed at four corners of a rectangle to virtually increase the size of the diaphragm, and to control the directivity in the range down to the low frequency range.
in such a prior art, system the directivity is determined by the disposition intervals of the speaker units. Therefore, to have directional directivity in the low frequency range, it is enough to widen the disposition intervals of the speaker units in the horizontal direction and the vertical direction. As the intervals become wider, however, the sound pressure level of the side lobes increase in the directivity pattern, and the side lobes are generated in relatively lower frequency range, thereby deteriorating the directivity. In a speaker system, moreover, it Is desirable for the sound pressure frequency characteristic be axis-symmetrical with respect to its central axis, and hence a speaker unit for sound reproduction in medium and high frequency ranges is disposed in the central part of the area surrounded by the Four speaker units for sound reproduction in the low frequency range. Herein, to control the directivity characteristics in the medium and high frequency ranges, a horn speaker is generally used for the speaker for sound reproduction in the medium and high frequency ranges. To control the directivity in the frequency range from the medium frequency range by the horn speaker, the control band of the directivity is determined by the size of the horn mouth. Since the mouth is considerably large, the disposition intervals of the speaker units for sound reproduction in the low frequency range must be set larger than the diameter of the horn mouth. Therefore, when a speaker system is composed by dividing the frequency band into a low frequency band and a medium and high frequency band by a dividing network, the effects of side lobes appear in the reproduction band due to the directivity of the speaker for sound reproduction in the low frequency range, and a large disturbance occurs in the sound pressure frequency characteristic near the crossover frequency with respect to the medium and high frequency range. Therefore, to obtain favorable directivity as the speaker system, the crossover frequency of the dividing network must be set in a to a very low frequency band so as to be free from effects of the side lobes of the speaker for sound reproduction in the low frequency range. However, in the horn speaker for sound reproduction in the medium and high frequency range, the sound pressure level is lowered in the low frequency range because the acoustical load of the horn is not applied, and the distortion increases, so that sufficient quality reproduction sound is not obtained from the speaker system.